The present invention relates to a flow rate measuring device for measuring a flow rate of a fluid flowing in a flow path, and more specifically, relates to a flow rate measuring device capable of measuring the flow rate with high accuracy by reducing a change of output characteristics caused by a physical property change of the fluid.
Heretofore, in order to measure a flow rate of a fluid such as gas flowing in a flow path (hereinafter, referred to as “a fluid to be measured”), a thermal flow rate measuring device that measures the flow rate of the fluid to be measured, based on a change of a temperature distribution in the flow path, is used.
FIG. 13 (a) and FIG. 13 (b) are schematic views for explaining the change of the temperature distribution in the thermal flow rate measuring device. FIG. 13 (a) shows a temperature distribution in a state where the fluid to be measured does not flow, and FIG. 13 (b) shows a temperature distribution in a state where the fluid to be measured flows.
As shown in FIG. 13 (a), in a state where the fluid to be measured does not flow, the fluid to be measured, which is present on a periphery of a micro heater 181, is heated by the micro heater 181. Therefore, in the micro heater 181, a uniform temperature distribution is generated over a thermopile 182 disposed on an upstream side of the micro heater 181 and a thermopile 183 disposed on a downstream side of the micro heater 181.
When the fluid to be measured flows in an arrow direction of the drawings in such a state, as shown in FIG. 13 (b), a temperature distribution on the periphery of the micro heater 181 is biased to a downstream side of the flow of the fluid to be measured, that is, to the thermopile 183 side. Therefore, on the thermopile 182, a lower temperature than in the state where the fluid to be measured does not flow is detected, and on the thermopile 183, a higher temperature than in the state where the fluid to be measured flows is detected.
As described above, in the thermal flow rate measuring device, the flow rate of the fluid to be measured, which flows in the flow path, is calculated based on a difference between temperatures detected by the thermopile 182 and the thermopile 183, whereby it is made possible to measure the flow rate with high accuracy.
However, when a type or composition of the fluid to be measured is changed, a physical property such as thermal conductivity, specific heat, viscosity and density is also changed. Therefore, in the conventional thermal flow rate measuring device, there has been a problem that output characteristics are changed in response to the physical property of the fluid to be measured.
FIG. 14 (a) and FIG. 14 (b) are schematic views showing temperature distributions when Gas A and Gas B which have different physical properties, individually flow through a flow path 121 at a predetermined flow rate (L/min), and FIG. 15 is a graph showing relationships between the flow rates (L/min) of the Gas A and the Gas B, which are shown in FIG. 14 (a) and FIG. 14 (b), and output values (V) of the flow rate measuring device.
As shown in FIG. 14 (a) and FIG. 14 (b), even in the case where such fluids to be measured, which have the same flow rate, are flown through the flow path 121, such a temperature distribution on the periphery of the micro heater 181 is different between the Gas A and the Gas B which have different physical properties.
Therefore, as shown in FIG. 15, between the Gas A and the Gas B which have different physical properties, the output value (V) of the flow rate measuring device is changed even at the same flow rate, and a variation is increased with an increase of the flow rate of the fluid to be measured.
As described above, in the conventional thermal flow rate measuring device, in the case where the physical property of the fluid to be measured is changed, the output characteristics of the flow rate measuring device are changed, and accordingly, it has been difficult to measure the flow rate with high accuracy.
For such a problem as described above, Japanese Unexamined Patent Publication “Patent No. 4050857 (registered in Dec. 7, 2007) (Patent Document 1) and Patent Document 2: United States Patent Publication “U.S. Pat. No. 5,237,523 (registered in Aug. 17, 1993)” (Patent Document 2) disclose flow rate measuring devices, each of which includes a physical property sensor that detects the physical property of the fluid to be measured.
FIG. 16 is a top view showing a configuration of a micro flow sensor 207 including the flow rate measuring device disclosed in Patent Document 1, and FIG. 17 is a perspective view showing an exterior appearance of a flow rate measuring device 301 disclosed in Patent Document 2.
As shown in FIG. 16, in the micro flow sensor 207 of Patent Document 1, thermopiles 282 and 283 for measuring the flow rate and thermopiles 272 and 273 for detecting the physical property are disposed along four sides of a micro heater 281 on a substrate 205.
Specifically, with respect to the flow direction R of the fluid to be measured, the thermopile 282 for measuring the flow rate is disposed on an upstream side of the micro heater 281, and the thermopile 283 for measuring the flow rate is disposed on a downstream side of the micro heater 281. Moreover, the thermopiles 272 and 273 for detecting the physical property are respectively disposed on both ends of the micro heater 281 in a longitudinal direction (that is, a direction orthogonal to the flow direction R).
Moreover, as shown in FIG. 17, in the flow rate measuring device 301 of Patent Document 2, a flow rate sensor 308 is disposed on an inner wall of a main flow path 321, and a physical property sensor 307 is disposed inside a cell 336 divided from the main flow path 321.
According to Patent Document 1 and Patent Document 2, the physical property of the fluid to be measured is calculated based on an output value of the physical property sensor, and the flow rate of the fluid to be measured is corrected by using the calculated physical property, whereby the change of the output characteristics of the flow rate measuring device, which results from such a physical property change of the fluid to be measured, can be reduced.
Here, the flow rate sensors and the physical property sensors have specific detection ranges. If the flow rate of the fluid to be measured goes out of the detection ranges, the measurement accuracy is lowered or it may be impossible to measure the flow rate. Therefore, in order to enhance the measurement accuracy of the flow rate measuring device, optimum flow rates corresponding to the detection ranges of the flow rate sensor and the physical property sensor may be individually controlled.
However, in the technology of Patent Document 1, a configuration is formed, in which the thermopiles 282 and 283 for detecting the flow rate and the thermopiles 272 and 273 for detecting the physical property, both being provided on the substrate 205, are disposed in the same flow path. Therefore, the optimum flow rates cannot be individually controlled for each of the flow rate sensor and the physical property sensor.
Therefore, in the technology of Patent Document 1, the output characteristics of the physical property sensor (thermopiles 272 and 273 for detecting the physical property) change with the flow rate of the fluid to be measured. Accordingly, for the calculated physical property, a correction corresponding to the flow rate of the fluid to be measured may be performed. That is to say, as shown in the following Expression (1), the physical property (coefficient) detected for correction of the flow rate output value, may be corrected by using the flow rate output value before correction.[Expression 1]Flow rate output value after correction=flow rate output value before correction×(coefficient corresponding to physical property×flow rate output value before correction)  Expression (1)
Hence, in the technology of Patent Document 1, an error by the physical property cannot be corrected completely, and accordingly, the flow rate of the fluid to be measured cannot be measured with high accuracy.
Moreover, in the technology of Patent Document 2, there is formed a configuration in which the main flow path 321 and the cell 336 is in fluid communication with each other by one pipe. Accordingly, an inflow and outflow of the fluid to be measured to and from the cell 336 are stagnated, and efficient substitution cannot be performed for the fluid to be measured in the cell 336.
Therefore, in the technology of Patent Document 2, for example, in the case where the physical property of the fluid to be measured is changed, the fluid to be measured which flows on a periphery of the physical property sensor 307 disposed in the cell 336, and the fluid to be measured which flows on a periphery of the flow rate sensor 308 disposed in the main flow path 321, differ in physical property from each other. Hence, an appropriate physical property cannot be detected by the physical property sensor 307.
Hence, in the technology of Patent Document 2, an accurate correction by the physical property cannot be made, and accordingly, the flow rate of the fluid to be measured cannot be measured with high accuracy.